<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>

</body>
<script>
    function fun(n, o) {
        console.log(o)
        return {
            fun: function (m) {
                return fun(m, n);
            }
        };
    }
    var a = fun(0);
    console.log(a);
    a.fun(1);
    a.fun(2);
    a.fun(3);
    var b = fun(0).fun(1).fun(2).fun(3);
    var c = fun(0).fun(1);
    c.fun(2);
    c.fun(3);

</script>

</html>